LONDON — The drug trial of a former British "X Factor" judge collapsed on Monday after the judge said the star prosecution witness, an undercover reporter known as the "Fake Sheik," had lied under oath.

The Rupert Murdoch-owned Sun on Sunday tabloid reported last year it had caught Tulisa Contostavlos acting as go-between in a deal to sell cocaine to a reporter posing as a film producer.

The scoop was the work of Mazher Mahmood, a journalist famed for pulling off undercover stings while disguised as a wealthy Arab businessman.